The present invention relates to an illuminator provided with a light guide plate, and to a liquid-crystal display device using the illuminator.
Recently, portable electronic apparatuses represented by a portable information terminal, a portable telephone, etc. have been made smaller in size and lower in price. As a result, such portable electronic apparatuses have come into wide use.
Specific examples of such a portable electronic apparatus include a reflection type liquid-crystal display device which is effective in reducing power consumption, and a liquid-crystal display device using a reflection type liquid-crystal display element and a front-lighting illuminator.
As the performance required of this illuminator, the irradiation quantity of the light which irradiates a liquid-crystal panel has to be large and the whole surface of the liquid-crystal panel has to be irradiated uniformly. The enhancement of the irradiation quantity of light is achieved easily by the increase of the quantity of the light radiated from a light source. However, such a method cannot be regarded as practical because it is accompanied by the increase of power consumption.
As the background art concerning such an illuminator, JP-A-10-188636 discloses a method in which a light source is disposed in an end portion of a light guide plate made of a transmissive material and small protrusion portions for taking the light out toward a liquid-crystal display device are formed on the lower surface of the light guide plate, as shown in FIG. 1.
In addition, JP-A-11-53918 discloses another method in which small protrusion portions (or small recess portions) for reflecting the light entering a light guide plate toward a liquid-crystal display device are formed on the upper surface of the light guide plate, as shown in FIG. 1.
Further, JP-A-11-72787 discloses a further method in which small protrusion portions or small recess portions for transmitting the light entering a light guide plate toward a liquid-crystal display device are formed at random on the lower surface of the light guide plate.
The following properties are required of an illuminator for use in the condition that it is disposed in front of a liquid-crystal display device.
(1) Haze (turbidity or cloudiness) of a light guide plate is low.
(2) The surface reflectivity is low.
(3) The light entering eyes directly from the light guide plate used in the illuminator is less.
(4) The exiting angle of the light made to go out from the lower surface of the light guide plate used in the illuminator is small.
(5) No moire pattern is produced.
In the above-mentioned background art disclosed in JP-A-10-188636, however, the exiting angle of the light made to go out from the lower surface of the light guide plate becomes large due to the sectional shape of each of the protrusion portions. Therefore, there is such a problem that not only is the light entering the liquid-crystal display device less, but also the protrusion portions are arranged regularly so that moire is produced easily.
On the other hand, in the background art disclosed in JP-A-11-53918, the aforementioned problem can be improved somewhat, but the light entering eyes directly from the light guide plate is apt to be generated due to the shape of each of the protrusion portions (or recess portions). As a result, a problem in visibility as a liquid-crystal display device still remains. In addition, moire is produced easily due to the arrangement of the protrusion portions.
Further, in the background art disclosed in JP-A-11-72787, the exiting angle of the light made to go out from the lower surface of the light guide plate becomes large due to the sectional shape of each of the protrusion portions. Accordingly, not only is the light entering the liquid-crystal display device less, but also it is difficult to dispose a large number of small protrusion portions or small recess portions irregularly.
The present invention has been developed to solve the foregoing problems. It is an object of the present invention to provide an illuminator which can enhance the irradiation quantity of the light which irradiates a liquid-crystal display device without increasing the quantity of the light radiated from a light source; a method for manufacturing the illuminator; and a liquid-crystal display device using the illuminator.
In order to attain the foregoing object, an illuminator disposed in front of a liquid-crystal cell according to the present invention is constituted by a light guide plate and a light source disposed on one of side surfaces of the light guide plate. This light guide plate includes an incidence surface on which the light from the light source is incident, and a light transmission surface through which the incident light on the light guide plate is made to exit to the liquid-crystal cell. In addition, a plurality of dots each constituted by a small recess portion or a small protrusion portion for reflecting the light incident on the incidence surface toward the light transmission surface are formed on the surface opposite to the light transmission surface. Each of the dots has a substantially V-shape in section, and an inclination angle of the section is in a range of from 35 to 43xc2x0. A vertex angle of each of the dots is set to be in a range of 70.6xc2x12.50.
In addition, according to the present invention, each of the dots is substantially rectangular in plan shape, and each of the dots is set to have a short-side length in a range of from 0.002 to 0.05 mm and a long-side length in a range of from 0.002 to 0.2 mm.
Then, the dots each having such a shape are disposed at random on the surface opposite to the light transmission surface of the light guide plate.
Further, according to the present invention, the light guide plate constituting the illuminator includes an incidence surface on which the light from the light source is incident, and a light transmission surface through which the incident light on the light guide plate is made to exit to the liquid-crystal cell, and a plurality of dots each constituted by a small recess portion or a small protrusion portion for reflecting the light incident on the incidence surface toward the light transmission surface are formed on the surface opposite to the light transmission surface.
Then, not smaller than 95% of the whole area of the surface on which the dots are formed is sectioned into 0.25 to 1 mm2 square areas, and the dots are disposed in each of the square areas so that a function G(R) which is obtained by taking a weighted average of a radial distribution function g(R) obtained for each of the dots in accordance with an arrangement relationship of the dots, and which is obtained by approximating the weighted average by a least squares method satisfies a relation of 0 less than S1/S2 less than 0.2 in a range of R/R0=3 to 6.
Provided that R designates a distance from a central position of one dot to a central position of another dot; R0, a value obtained by dividing a length of one side of the square area by a square root of the number of the dots existing in the square area; S1, a value obtained by integrating a difference between G(R) and an average value of G(R) with R/R0 which is in a range of from 3 to 6; and S2, a value obtained by integrating the average value of G(R) with R/R0 which is in a range of from 3 to 6.
In addition, each of the dots is disposed so that the function G(R) is substantially 0 in a range of R less than (short-side length of dot)xc3x972, at least two peaks exist in the function G(R), and two peaks each of which is at least twice as large as the average value of the function G(R) exist in a range of R/R0=3 to 6.
Moreover, according to the present invention, an oxide film is formed on a surface of a silicon substrate having a predetermined crystal plane and a resist film is formed on the oxide film so that each dot to be formed on the light guide plate has a V-shape in section and an inclination angle of the section is in a range of from 35 to 43xc2x0. Then, a dot pattern is formed on the oxide film with the resist film serving as a mask. Then, after anisotropic etching is given to the silicon substrate with the oxide film serving as a mask, a metal film is further formed on the silicon substrate. Further, the metal film is stripped off so as to produce a stamper or a replica thereof. Dots are transferred onto a surface of a film or plastic by use of the stamper or the replica. Thus, a light guide plate having such dots is formed.
A liquid-crystal display device according to the present invention has an illuminator which has such a light guide plate, a liquid-crystal display portion and a control portion. The illuminator is disposed in front of the liquid-crystal display portion so that external light is transmitted through the illuminator and enters the liquid-crystal display portion. The quantity of light with which the illuminator irradiates the liquid-crystal display portion is controlled by the control portion in accordance with the quantity of the external light.
Further, a portable electronic apparatus using the liquid-crystal display device according to the present invention has a light receiving portion. The illuminator is controlled by use of the quantity of the external light received by the light receiving portion so that the luminance of the liquid-crystal display portion is made substantially constant.
Furthermore, the portable electronic apparatus also has a signal receiving portion. The illuminator is controlled by the control portion with a signal supplied to the signal receiving portion as a trigger, so that the liquid-crystal display portion is irradiated with light in accordance with the external light entering the light receiving portion. Thus, the luminance of the liquid-crystal display portion is made substantially constant.